1. Field of the Invention
The present invention relates to an injection molded article used with a photosensitive material, and more particularly to an injection molded article having various characteristics, for example, oil resistance, resistance to organic solvents, heat resistance, tasteless property, non-odorous property, non-toxicity, moisture absorption resistance, dimensional stability, surface hardness, and fracture resistance.
2. Description of the Related Art
A photosensitive material has a membrane consisting of photographic emulsion and formed on a support member by a method of coating or deposition. The photosensitive material is classified into a photographic film, a dry plate, a photographic paper and so forth in accordance with a kind of the support member. The photographic film uses a resin film as the support member. As the photographic film, there are, for example, various roll films having a size of from 8 mm to 1,000 mm, a cabinet of 4 cm.times.5 cm and various sheet films having a size of B5, A4, B4 and A4. These photosensitive materials are sold in a state that they are light-tightly contained in a package constituted of injection molded articles made of a thermoplastic resin including a light shielding material.
Homopolystyrene resin being GP polystyrene resin (hereinafter GPPS resin), in which only styrene monomer is polymerized, has good dimensional stability. Moreover, the homopolystyrene resin does not have any taste, any odor and any toxicity. Thus, the homopolystyrene resin is manufactured in great quantities for transparent containers. However, the homopolystyrene resin tends to be damaged by some of oils and organic solvents, and does not have sufficient resistance to climate and heat. Further, it tends to be electrostatically charged and is inferior in resistance to fracture. Therefore, rubber including polystyrene resin (generally called high impact polystyrene resin or impact resistance polystyrene resin, hereinafter HIPS) is used for an injection molded article requiring light shielding property and including a light shielding material of carbon black and so forth (refer to Japanese Patent Laid-Open Publication No. 1-312538). In the HIPS resin, for example, synthetics rubber of butadiene rubber and so on is graft copolymerized for the styrene monomer.
The synthetic rubber is included in the HIPS resin. A rubber-like material of such synthetic rubber and so forth gives bad influences (fog, sensitization and so on) for the photographic property of the photosensitive material. Accordingly, content of the rubber-like material is restricted to 1-12% (refer to Japanese Patent Laid-Open Publication No. 6-313948).
By the way, polypropylene-based resin was used for a container body of a film cassette container. As to the container body, the dimensional stability of the injection molded article is not required so much, however, it is required to be superior in moistureproofness, transparency, rigidity, appearance, molding cycle and impact strength (refer to Japanese Patent Publication No. 2-38939). In the polypropylene-based resin, the content of propylene resin having high melt index (16-80 g/10 min) is 70 wt. % or more.
Moreover, high-pressure ramification low-density polyethylene resin (hereinafter LDPE resin) having high melt index (7-40 g/10 min) has been used for a cap of the film cassette container (refer to Japanese Patent Publication No. 6-29953). As to the cap, the dimensional stability of the injection molded article is not required so much, however, it is required to be superior in moistureproofness, suitable rigidity, molding cycle, impact strength, and fitting property for the container body.
However, when the injection molding is performed at rapid cycle, using the above resin composition and a multi-cavity die having a plurality of same cavities and cores, number of which is more than ten, dimensions of the injection molded articles become different according to molding position. In consequence, fitting with the container body becomes worse. Further, the moistureproofness becomes worse and fitting strength reduces. Therefore, there arise a problem in that the cap may be easily separated from the container body by being dropped from a hand and by pressing with heavy thing during transportation. If the cap comes off, the moistureproofness is not kept. Further, photographic property of the photographic film is not kept and the photographic film cassette are not protected. In order to solve this problems, it is considered that the molding cycle is lengthened so as to take out the injection molded article from the die after it was completely cooled. However, in this case, the productivity is greatly decreased.
The rubber-like material and the light shielding material lower the flowability of the resin so that the injection moldability becomes worse. Further, weld lines occur on a surface of the injection molded article made of the resin composition so that its appearance is damaged. Moreover, upon adding the rubber-like material and the light shielding material, the elasticity of the resin lowers so that the injection molded article made of the resin composition including them can not obtain sufficient physical strength. Accordingly, it is difficult to maintain the wear resistance and the resistance to scuffing at high level.
As to the injection molded article made of the conventional high-impact polystyrene resin, heat distortion temperature thereof is low. Accordingly, when the package for the photosensitive material, which is used under the sun light such as a lens-fitted film unit, a camera, a photo film cassette made of resin, a sheet film unit and so forth, is made of such injection molded articles, the package is distorted by heat of the sun light, impact from outside and friction force. Thus, there is possibility that loading to the camera or advancement of the photographic film becomes impossible. Moreover, even if photographing can be performed, there is possibility that fatal troubles for the photosensitive material occur, for example, the light shielding property is damaged by the distortion of the package, the photographic fog occurs on the photosensitive material, the flaw is generated on a photographic emulsion face during film advancement, the dust adheres to the photographic emulsion, and so forth. Furthermore, the resistance to chemicals is inferior and the distortion is caused by benzene, toluene, acetone, ethyl acetate and so forth. Therefore, there arises a problem that it is impossible to keep the light shielding property, the dimensional accuracy and so forth.